1. Field of the Invention
The present invention relates generally to superhard cutting elements, and more specifically to substantially planar polycrystalline diamond compact cutting elements comprising a polycrystalline diamond table formed and bonded to a supporting substrate or backing during formation of the cutting element.
2. State of the Art
Polycrystalline diamond compact cutting elements, commonly known as PDC's, have been commercially available for over 20 years. PDC's may be self-supporting or may comprise a substantially planar diamond table bonded during formation to a supporting substrate. A diamond table/substrate cutting element structure is formed by stacking into a cell layers of fine diamond crystals (100 microns or less) and metal catalyst powder, alternating with wafer-like metal substrates of cemented tungsten carbide or other suitable materials. In some cases, the catalyst material may be incorporated in the substrate in addition to or in lieu of using a powder catalyst intermixed with the diamond crystals. A loaded receptacle is subsequently placed in an ultra-high temperature (typically 1450.degree.-1600.degree. C.) ultrahigh pressure (typically 50-70 kilobar) diamond press, wherein the diamond crystals, stimulated by the catalytic effect of the metal power, bond to each other and to the substrate material. The spaces in the diamond table between the diamond to diamond bonds are filled with residual metal catalysis. A so-called thermally stable PDC product (commonly termed as TSP) made be formed by leaching out the metal in the diamond table. Alternatively, silicon, which possesses a coefficient of thermal expansion similar to that of diamond, may be used to bond diamond particles to produce a Si-bonded TSP. TSP's are capable of enduring higher temperatures (on the order of 1200.degree. C.) without degradation in comparison to normal PDC's, which experience thermal degradation upon exposure to temperatures of about 750.degree.-800.degree. C.
While PDC and TSP cutting elements employed in rotary drag bits for earth boring have achieved major advances in obtainable rate of penetration while drilling and in greatly expanding the types of formations suitable for drilling with diamond bits at economically viable cost, the diamond table/substrate configurations of state of the art planar cutting elements leave something to be desired.
First, bending attributable to the loading of the cutting element by the formation may cause fracture or even delamination of the diamond table from the substrate. It is believed that such degradation of the cutting element is due at least in part to lack of sufficient stiffness of the cutting element so that, when encountering the formation, the diamond table actually flexes due to lack of sufficient rigidity or stiffness. As diamond has an extremely low strain rate to failure, only a small amount of flex can initiate fracture.
In addition to the aforementioned shortcoming, state of the art PDC's often lack sufficient diamond volume to cut highly abrasive formations, as the thickness of the diamond table in state of the art cutting elements is not adequate for such formations.
Furthermore, the use of single-thickness diamond tables on cutting elements travelling in overlapping or partially overlapping circular paths may result in unnecessary redundancy of diamond volume in the overlap area.
The benefits of a multi-thickness diamond table, which produces a kerfing action during drilling as the thicker portions wear less than the thinner portions, have been recognized. Kerfing may generally be defined as grooving, scoring or scribing a formation, and more specifically as relieving a formation, ideally in a ratio of at least one to one in groove height to width. However, all such prior art PDC configurations (see, for example, U.S. Pat. Nos. 4,784,023 and 5,120,327) employ parallel linear interleaved ridges of diamond and substrate extending across the cutting element. However, the use of several parallel thick ridges on the relatively small surface of a typical PDC cutting element may fail to provide any kerfing benefit whatsoever in terms of energy expended to drill in harder or more abrasive formations.
Another PDC cutting element structure which affords a multiple-depth diamond table is disclosed in European Patent Specification Publication No. 0 322 214 B1. This structure's substrate ridges resemble a "bulls-eye" pattern in one embodiment, and a spiral pattern in another. While allegedly providing curved cutting ridges as the cutting element wears, wear of such ridges causes the primary contact points between the cutting element and the formation to migrate rapidly laterally, so that a deep kerf or cleft in the formation at a substantially constant radial location at the bottom of the borehole is never effected.
Yet another PDC cutting element structure which affords a multiple-depth diamond table is disclosed in U.S. Pat. No. 4,984,642. In this instance, the ridges or grooves are actually formed in the surface of the diamond table rather than at the boundary between the diamond table and the underlying, supporting substrate. However, this structure possess the same deficiencies as the previously-referenced patents employing interleaved ridges of diamond and substrate extending across the substrate element.
U.S. patent application Ser. No. 08/016,085, filed Feb. 10, 1993 and assigned to the assignee of the present invention, discloses the use of a substrate with radially-oriented lands to redistribute stresses at the diamond/substrate interface, which structure also provides a multiple-depth diamond table.
Still another PDC cutting element structure which affords a diamond table having either an increased or reduced thickness in the center of the cutting element is disclosed in U.S. Pat. No. 4,954,139. In this instance, while the diamond table may indeed be thicker as it approaches the center of the cutting element, the periphery or skirt of the diamond table which initially encounters the formation is of reduced thickness, and thus inherently less stiff and more flexible.